Wednesday, December 14, 2011

Zombie Ants Go Marching

Imagine a sudden insatiable need to climb shatters the monotony of your agonizingly redundant workday. As you climb higher and higher a sharp pulsating pain begins to emanate from the back of your head. Gaining in intensity which each thump, the pain has now become unbearable. Succumbing to the misery, with your last dying breath grasp onto the nearest structure within arms reach. After a period of time a fruiting body erupts from your corpse releasing spores hoping to find another victim. This scenario is a terrifying possibility for unlucky arthropods that have been selected as the specific host for pathogenic fungi belonging to the genus ophiocordyceps. One such relationship exists between the carpenter ant Camponotus leonardi and fungus Ophiocordyceps unilateralis

Sensationalized through articles with titles such as “Fungus Makes Zombie Ants Do All the Work” and a feature in the incredibly popular documentary series Planet Earth, carpenter ants and the fungus have become the poster child for these particular host-parasite systems. What makes the fungus so interesting is the parasitic relationship in which the fungus has the ability to manipulate the behavior it’s host carpenter ant.

The worker carpenter ants become infected when they come into contact with spores of the fungus. Contact is usually made on the forest floor and it is hypothesized that as a defense mechanism the ants only make the descent when there is no other way to traverse from plant to plant. The period of infection has been seen to be as short as 3-6 days. Presumably as the fungus begins to establish itself within the ant’s carapace the behavioral modification sets in.

By some unknown mechanism infected ants will voluntarily leave the colony and begin climbing as to not spread the fungal infection but it has also been observed that workers will actually carry out infected individuals unable to leave under their own power. As the infection worsens the ailing ant fastens itself at various places all over a plant but usually on the undersides of leaves. Fastening is accomplished through a biting behavior seen as an extended phenotype of the fungus and will typically immediately precede death. The clenching of the ants mandible keeps the corpse in place as the fungal mycelia continue to propagate and also produce adhesives that more permanently bind the ant in place.

Once the growing fungus is reproductively ready, a stroma will burst through the back of ant’s head. The spore producing perithecia then grows from once side of the stroma. Spores released by the fungus are too heavy to be wind dispersed and so just fall to the forest floor where they produce secondary spores that infect new hosts as they come into contact.

What is remarkable is that large aggregates of dead ants have been observed in nature and deemed graveyards. In these graveyards researchers have seen as many as 14 dead ants per square meter transect. During these research expeditions researchers also wanted to count how many live ants could be found in areas higher densities of dead ants. Despite extensive scouring of the areas of interest only 2 live worker ants were found compared to roughly 2500 dead ones.

The carpenter ants were seen to only establish colonies in forest canopy with a network of aerial trails criss crossing through the trees. Very rarely were foraging trails seen traversing the forest floor. As mentioned before, researcher hypothesized that the carpenter ants have developed this behavior of avoiding the forest floor and graveyards as a defensive mechanism against infection by fungus. Perhaps if a nearby but aerially inaccessible tree was seen to resourcefully profitable a colony would risk infection in order to colonize the tree.

At the moment there are no pathogenic fungi like ophiocordyceps currently targeting humanity, but people should still maintain awareness. There could be a time when people you are close to become mindless zombies marching to their doom.

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This blog represents the work of students from one of the few courses devoted to Eukaryotic Microbiology as a whole. This is a new experiment for this course that we hope will be a successful fusion of student learning and science communication. Enjoy, but play nice, comments are welcome but mean-spirited comments will be deleted.